Vent holes are often provided in housings of various devices, as exemplified by: automobile electric components such as headlamps, rear lamps, fog lamps, turn lamps, motors, various pressure sensors, and pressure switches; cameras; videos; information terminals such as mobile phones; electric shavers; electric toothbrushes; and lamps for outdoor use. The main purpose of providing a vent hole in a housing of a device is to provide ventilation between the inside and outside of the device, and thus to avoid excessive increase in the internal pressure associated with the fact that the temperature inside the housing of the device is increased by operation of the device. Also, a vent hole is provided in a casing of a battery for the purpose of emitting a gas generated during operation of the battery.
In order to prevent entry of water or dust through a vent hole provided in a housing of a device, a gas-permeable filter is attached to the vent hole in some cases. A porous membrane made of a polyolefin resin or a fluorine resin is often used as a gas-permeable filter. In particular, a porous membrane formed by stretching polytetrafluoroethylene (hereinafter, referred to as “PTFE”) and having a microporous structure is known as a gas-permeable filter excellent in water repellency. Depending on the usage environment, a gas-permeable filter may contact sebum, a surfactant, an oil, or the like. Even when a stretched PTFE porous membrane excellent in water repellency is used as a gas-permeable filter, entry of a liquid having a low surface tension cannot be fully prevented. Accordingly, depending on the intended use, a gas-permeable filter is subjected to oil-repellent treatment using a treating agent including a fluorine-containing polymer.
It is well-known that a fluorine-containing polymer having a linear perfluoroalkyl group (hereinafter, “linear perfluoroalkyl group” may be referred to as “Rf group”) having eight or more carbon atoms is suitable for providing oil repellency. Rf groups having eight or more carbon atoms have a significantly higher crystallinity than Rf groups having a smaller number of (e.g., six or less) carbon atoms. The high crystallinity is considered to contribute to exerting excellent oil repellency. It is also known that, due to the high crystallinity, a treating agent having an Rf group having eight or more carbon atoms allows for a large receding contact angle (which is one of the dynamic contact angles, along with an advancing contact angle). The receding contact angle becomes larger with increasing crystallinity, and sharply increases as the number of carbon atoms changes from six to eight. For these reasons, usual methods for providing oil repellency to a gas-permeable filter use a treating agent including a fluorine-containing polymer having an Rf group having eight or more carbon atoms.
It is also known that oil repellency is provided to a gas-permeable filter using another treating agent together with the above treating agent. For example, JP H7(1995)-126428 A (Patent Literature 1) discloses treating a gas-permeable filter with a treating agent that includes a fluorine-containing polymer having an Rf group and that also includes a fluorine resin whose main chain has a fluorine-containing alicyclic structure (claim 1 etc.). The fluorine resin having a fluorine-containing alicyclic structure has properties excellent for film formation, and can be obtained, for example, by polymerization of perfluoro(2,2-dimethyl-1,3-dioxole) (paragraphs [0009] and [0011]). Patent Literature 1 teaches that the number of carbon atoms in the perfluoroalkyl group is 4 to 16, and particularly preferably 6 to 12 (paragraph [0023]). However, in the description of Examples, a fluorine-containing polymer having perfluoroalkyl groups having nine carbon atoms on average is used as in the usual methods describe above (paragraphs [0049] and [0050]; Examples).